The present invention relates to a supported chromium oxide catalyst, in particular a catalyst system comprising an inorganic support, a chromium oxide and a metallocene compound, and a method for the preparation of said catalyst system. The catalyst produces branched polyethylenes without the use of a copolymer, and the molecular weight can be controlled by feeding hydrogen.
To obtain linear polyolefins having desired properties, different catalyst systems in combinations with controlled polymerisation conditions are used. A typical supported chromium catalyst that is extensively used in commercial polymerisations of ethylene is formed by depositing a chromium compound onto a support, which is then oxidised. The oxidised catalyst precursor may be introduced as such into a polymerisation reactor, where it will be reduced in situ by the olefin monomers to its active catalytic state. It is also well known that oxidised chromium compounds may be prereduced by suitable reagents, such as hydrogen or carbon- monoxide (J. P. Hogan, J.Polym. Sci., PtA-1, 8, 2637 (1970), and references therein). The resulting chromium surface species are generally accepted to be highly reactive. A prereduced chromium-based catalyst will produce polyethylenes having a broad molecular weight distribution (MWD) and mainly long, straight chains. Such catalysts are not hydrogen-sensitive.
Another commercially important class of catalysts used to polymerise xcex1-olefins are the cyclopentadienyl transition metal catalysts, usually termed metallocene catalysts. Metallocenes contacted with Lewis acids will give effective polymerisation catalysts that produce linear polyethylenes having a narrow molecular weight distribution (MWD). Such catalysts are sensitive to the presence of hydrogen.
By adding comonomers and optionally hydrogen during the catalysed polymerisation reactions the density and chain branching of the produced polymer can be controlled. In the prior art efforts have been made to develop combined catalyst system that produce short chain branched polyethylenes, without the addition of comonomers during the polymerisation reactions.
U.S. Pat. No. 4,735,931 discloses a catalyst system prepared by first depositing a chromium compound on a predominantly silica support, activating said catalyst in an oxygen-containing atmosphere and thereafter subjecting the thus activated composition to reduction with carbon monoxide. The resulting chromium catalyst composition is then precontacted with a cocatalyst selected from trialkyl boron and dialkyl aluminium alkoxide compounds, preferably triethyl borane, and thereafter contacted with ethylene. When used in olefin polymerisations this catalyst provides in situ generation of comonomers, resulting in tough, flexible, mainly pure polyethylene resins.
U.S. Pat. No. 5,032,651 teaches the use of catalyst mixtures of two transition metal catalysts. One of the catalysts comprises chromium oxide supported on an aluminophosphate, and the other one comprises a xcex2-stabilized tetrahydrocarbyl zirconium compound supported on an inorganic material. The catalysts may be premixed before use, or they may be fed separately to the reactor. Olefinic polymers exhibiting high environmental stress crack resistance (ESCR) are produced.
EP 206794 describes an olefin polymerisation catalyst comprising a support contacted with a Group 4b, 5b or 6b metallocene and an aluminoxane.
EP 088 562 discloses a modified polymerisation catalyst comprising a silica support with deposited chromium. Following oxidation in dry air, the chromium is modified by being contacted with a transition metal compound of Ti, V or Cr, preferably Ti. Only the use of bis-toluene titanium is exemplified, and the obtained polyethylenes have a substantial degree of branching and a medium or broad molecular weight distribution.
U.S. Pat. No. 3,378,536 discloses a process for the polymerisation of ethylene by the use of a two-component catalyst system consisting of (a) a chromium compound deposited on e.g silica, where the chromium being activated in an oxygen-containing gas at a high temperature and then reduced with CO; and (b) chromium or vanadium arene where the arene is an aromatic, optionally substituted C6 ring. The two catalyst components are preferably fed separately to the polymerisation reactor.
It is well known to a person skilled in the art that the various processing techniques used in the manufacturing of final articles from polyethylene resins require polyethylene grades having specific rheological properties. For instance, in the extrusion blow moulding of containers polyethylenes having a broad MWD and long straight chains are typically used, while in film blowing lower density polyethylenes are preferred.
It is an object of the present invention to provide a novel catalyst system that, produces a branched polyethylene from ethylene without any addition of comonomer during the polymerisation reaction. A further object is to control the polymerisation reaction to produce polyethylenes having a desired density and molecular weight. Another object is to obtain polyethylene resins suitable for use blow moulding and film blowing processing.
It has now been found that a catalyst system comprising a prereduced chromium on silica catalyst that have been contacted with a metallocene compound fulfils the requirements above. The novel catalyst system produces a branched low density polyethylene polymer without any added comonomer. The density and molecular weight (and hence the melt flow index) of the polymer can be controlled by the addition of hydrogen to the reactor.
The present invention thus provides a catalyst system for the polymerisation of ethylene, comprising chromium oxide supported on an inorganic support, wherein
a) said support being a particulate inorganic oxide;
b) the chromium of said chromium oxide being in a reduced oxidation state, and comprising
c) a metallocene compound having the formula:
xe2x80x83Cp2ZrRxe2x80x2Rxe2x80x3
wherein each Cp, being equal or different, is an unsubstituted or substituted cyclopentadienyl compound, and Rxe2x80x2 and Rxe2x80x3, independent of each other, are selected from the group comprising alkyls having 1 to 6 carbon atoms, unsubstituted or substituted benzyl, and phenoxy substituted with alkyls having 1 to 6 carbon atoms, and Rxe2x80x2 or Rxe2x80x3 may be a halide.
The invention also provides a method for the preparation of the catalyst system above, comprising the steps of:
a) calcining a support being a particulate, inorganic oxide selected from the group comprising alumina, silica, titania, zirconia, magnesia, and combinations thereof,
b) joining onto the surface of said support a chromium-organic compound to obtain a catalyst precursor,
c) subjecting said catalyst precursor to oxidising conditions to obtain the chromium in an oxidised state,
d) subjecting said catalyst precursor to reducing conditions to obtain a prereduced catalyst, thus
e) reducing the oxidised chromium to obtain the main part thereof in a bivalent oxidation state,
f) contacting said reduced catalyst with a metallocene compound having the formula:
Cp2ZrRxe2x80x2Rxe2x80x3
wherein each Cp, equal or different, is an unsubstituted or substituted cyclopentadienyl compound, and Rxe2x80x2 and Rxe2x80x3, independant of each other, are selected from the group comprising alkyls having 1 to 6 carbon atoms, unsubstituted or substituted benzyl, and phenoxy substituted with alkyls having 1 to 6 carbon atoms, and Rxe2x80x2 or Rxe2x80x3 may be a halide.
The catalyst system of the present invention comprises a supported reduced chromium/silica catalyst contacted with a metallocene compound. In general, metallocenes based on zirconium, hafnium and titanium are preferred as metallocene catalysts. By metallocenes are meant compounds in which a metal atom or ion or complex ion is xcex7-bonded by at least one ligand, e.g. by 1, 2 or 3 ligands or ligand components. The xcex7-bonding ligands in such catalysts may be simple unsubstituted cyclopentadienyl rings, but preferably they will be optionally substituted fused ring systems (e.g. indenyl ligands), substituted cyclopentadienyl rings, optionally substituted bridged bis-cyclopentadienyl ligands or optionally substituted bridged bis fused ring systems (e.g. bis indenyl ligands).
The catalyst support may be any oxide of metals from groups 2, 3, 4, 11, 12, 13 and 14 of the Periodic System of Elements. Preferred metal oxides are magnesia, alumina, titania, zirconia and silica. A particularly preferred catalyst support is silica. Such a silica support must contain not less than 90% pure silica, with the remaining part may consisting of other oxides, such as oxides of aluminium, zirconium, titanium, magnesium and phosphor. The support should consist of particles having preferably a spherical or spheroidal shape and a size from about 10 xcexcm to 150 xcexcm, more preferably from 20 xcexcm to 120 xcexcm, and a particle size distribution from narrow to broad within said ranges.
The chromium compound to be impregnated onto the silica support may be an inorganic chromium compound, such as chromium nitrate and chromium oxide, or an organic chromium compound, such as chromium acetate. Other chromium compounds are also useable. The chromium compound can be Joined with the support in any way known in the art, such as by coprecipitation with the silica support or addition to a silica gel after its formation, or after that it has been dried. A typical procedure of impregnation comprises the use of a water-soluble compound, or the use of an organochromium compound in an anhydrous hydrocarbon solution. The only precondition is that the chromium compound can be oxidized to a chromium oxide. The amount of chromium compound joined with the inorganic support must be sufficient to obtain from 0.1% to 10%, preferably from 0.5% to 2.0%, by weight of chromium, calculated as metallic chromium based on the weight of the supported chromium catalyst. When the impregnation is finished any possible remaining solvent is removed to obtain a dry solid.
Such solid chromium oxide/silica catalyst precursors are also commercially available from a number of producers. A closer description of their preparation is therefore regarded as being superfluous.
The obtained catalyst precursor must be activated before use. This is done by calcination in dry air or another oxygencontaining gas at temperatures in the range from 400 to 950xc2x0 C., preferably from 550 to 800xc2x0 C., during a period from 10 minutes to 24 hours, preferably from 2 to 20 hours. The oxidised catalyst precursor is conventionally subjected to reduction, preferably with carbon monoxide or a mixture of carbon monoxide and an inert component, such as nitrogen or argon. The reduction is normally performed at a temperature within the range from 300 to 500xc2x0 C., preferably from 350xc2x0 C. to 400xc2x0 C., during a period from 5 minutes to 48 hours, preferably from 1 to 10 hours. When the reduction is completed, the major part of the contained chromium should preferably be in a bivalent oxidation state. The final chromium catalyst particles should have a surface area from 200 to 600 m2/g, more preferably from 300 to 550 m2/g, and a pore volume from 1 to 3 cm3/g. The chromium oxide/silica catalyst in a reduced state, either as a dry powder or as a slurry in a dry oxygen-free hydrocarbon solvent, e.g. an alkane, must be stored in an inert ambience.
The present invention is not restricted to any particular procedure for the preparation of the chromium oxide/silica catalyst, and other methods than those described here may also be applicable.
The obtained chromium oxide/silica catalyst is contacted with a cyclopentadienyl-zirconium compound, hereinafter termed xe2x80x9czirconocenexe2x80x9d. Preferred zirconocenes can be represented by the simplified formula:
Cp2ZrRxe2x80x2Rxe2x80x3
wherein Cp designates cyclopentadienyl groups selected from unsubstituted cyclopentadienyl; cyclopentadienyl substituted with radicals selected from the group comprising unsubstituted and substituted linear, branched, cyclic or partially cyclic alkyl radicals, and annelated cyclic radicals, containing 1 to 20 carbon atoms; unsubstituted and substituted monocyclic or polycyclic aryl radicals which optionally also may contain hetero atoms; and aralkyl radicals. The substituents on the cyclopentadienyl ring may also form annelated structures comprising one or more fused benzene, naphtalene or cyclohexene rings, which optionally may contain hetero atoms. The substituents Rxe2x80x2 and Rxe2x80x3, equal or different, are selected from the group comprising alkyls having 1 to 6 carbon atoms, unsubstituted or substituted benzyl, and phenoxy substituted with alkyls having 1 to 6 carbon atoms. Preferably, Rxe2x80x2 and Rxe2x80x3 are independently selected from the group comprising methyl, benzyl or fenoxymethyl, and any combination thereof. One or Rxe2x80x2 and Rxe2x80x3 may also be a halide, preferably chloride.
A number of suitable metallocene compounds of the formula above are known in the art and have been recited in the literature. Particularly preferred metallocene compounds are embodied in the working examples.
Particularly good results are obtained when Cp represents cyclopentadienyl, and Rxe2x80x2 and Rxe2x80x3 are the same and are benzyl or methyl.
The zirconocene compound is joined with the activated prereduced chromium oxide/silica catalyst most conveniently by dissolving the zirconocene in a solvent, e.g. toluene, and impregnate the chromium oxide/silica catalyst with this toluene solution. Eventually, the solvent is removed by evaporation, preferably in an inert atmosphere, whereupon the catalyst is ready for use.
The final catalyst will contain both zirconium and chromium, preferably in a molar ratio of zirconium to chromium in the range from O.1:1 to not higher than 2:1. More preferably the catalyst contains zirconium and chromium in a molar ratio from 0.5:1 to 1:1.
Prior to polymerisation, the catalyst may optionally be prepolymerised with a minor amount of ethylene and/or an xcex1-olefin in accordance with methods well known in the art, before being fed continuously to the polymerisation reactor. Polymerisations can be performed in any conventional type of reactor, such as in a batch reactor or most preferably in a continuous reactor. The present catalysts are suitable for use in all types of olefin polymerisations, including gas phase and suspension polymerisations. In general, polymerisations are performed at temperatures below 110xc2x0 C., and at a total pressure in the range from ambient to 50 bar. Hydrogen is used to control the molecular weight, and consequently the melt flow index, of the polymer., whereas the catalyst determines the short chain branching on the polyethylene backbone and hence the density of the polyethylene.
The general polymerisation parameters mentioned above are well known in the art and further details concerning ethylene polymerisations should be superfluous. Typical polymerisation conditions are presented in the examples below.
It is realized that the present catalyst system has its highest catalytic activity when the substituents Rxe2x80x2 and Rxe2x80x3 of the zirconocene compound are pure hydrocarbyl radicals, in the examples embodied as benzyl or methyl. A person skilled in the art will understand that similar hydrocarbyl compounds are expected to have the same effect and that such compounds are contemplated by the present invention. The catalysts of this invention provide substantially higher activities than the prior art chromium/silica catalysts. When the substituents Rxe2x80x2 and Rxe2x80x3 contain hetero atoms, such as 0 and Cl, the present catalysts will get a reduced activity, typically lower than the activity of a comparative prior art chromium/silica catalyst. Thus, the activities of the catalysts of the present invention will be at their highest when the metallocene component is a pure hydrocarbyl zirconocene.
The present catalysts will have an activity that varies with the molar ratio between zirconium and chromium in the final catalyst. When the contained amount of zirconium is higher than the amount of chromium, the catalyst will have a lower catalytic activity than a comparative prior art chromium/silica catalyst. The present catalysts have their highest activity when the molar ratio between zirconium and chromium is about 0.5:1. This Indicates that only a minor amount of zirconocene is required to increase substantially the activity of a chromium/silica catalyst.
The catalytic activities of the present catalysts are also influenced by the concentration of hydrogen present in the polymerisation reactor. It has been found that an optimum level is about 1 bar of hydrogen.
The hydrogen level will also influence on the short chain branching of the produced polyethylenes. Analysis of polyethylenes polymerised with the present catalysts show that the obtained polymers have a higher amount of short chain branching than polyethylenes produced with a prior art chromium/silica catalyst. In particular, the amount of ethyl branches will increase. A higher level of hydrogen will increase the amount of short chain branching, in particular when using a catalyst having a ratio Zr/Cr of 0.5. By adjusting this ratio, as well as the hydrogen level inside the reactor, the morphology of the final resin can be controlled. Compared with the common used prior art chromium/silica catalysts, the catalysts of the present invention have a higher activity and will give a higher amount of short chain branching.
The present catalysts are preferably used to homopolymerise ethylene. However, it is also possible to use the present catalysts in copolymerisations of ethylene with a comonomer or a mixture of comonomers. Useful comonomers are all polymerisable xcex1-olefins having the general formula CH2xe2x95x90CHR, wherein R is a hydrocarbon radical containing 1-18 carbon atoms, preferably 1-10 carbon atoms. Examples of particularly preferred xcex1-olefins are propylene, 1-butene, 1-hexene, 4-methyl-l-pentene and 1-octene. However, the greatest achievements of the present catalysts are in homopolymerisations of ethylene.
The produced polyethylenes will have a density from 910 to  greater than 960 kg/m3, and a melt index from 0.01 to above 100 g/10 min, preferably from 0.1 to 60 g/l0 min (determined according to the method of ASTM 1238), depending on the polymerisation conditions, as explained above. More detailed specifications concerning the properties of the obtained polyethylenes are given in the examples.